Describe the Structural Variations of Vertebrate Kidney.
Structural Variations in Vertebrate Kidney.
The mammalian kidney is a compact organ with two distinct regions: cortex and medulla. The functional unit of the kidney is the nephron. Variations in nephron structure and physiology equip the kidneys of different vertebrates for osmoregulation in their various habitats. The ability of mammalian kidney to concentrate urine is closely related to the length of the loop of henle. Animals that produce the most highly concentrated urine have very long loops. Those that have a limited ability to concentrate the urine have short loops.
On the other hand, mammals with an intermediate ability to concentrate the urine (e.g., man, rabbit) have kidneys with both short and long loops. Among mammals, those able to excrete the most hyperosmotic urine, such as kangaroo rats and other mammals adapted to the desert, have exceptionally long loops of Henle that maintain steep osmotic gradients in the kidney.
This results in urine becoming very concentrated as it passes from cortex to medulla in the collecting ducts. In contrast, beavers, which rarely face problems of dehydration, have nephrons with very short loops, resulting in a dilute urine. Birds, like mammals, have kidneys with juxtamedullary nephrons that specialize in conserving water.
However, the nephrons of birds have much shorter loops of Henle than those typical of mammalian nephrons, and birds are unable to concentrate urine to the osmolarities achieved by mammalian kidneys. The kidneys of reptiles, having only cortical nephrons, produce urine that is, at best, isosmotic to body fluids.
Also, most terrestrial reptiles excrete nitrogenous wastes in an insoluble form known as uric acid, which helps to conserve water because it does not contribute to the osmolarity of the urine. In contrast to mammals and birds, freshwater fishes face the problem of excreting excess water because the animal is hyperosmotic to its surroundings. Freshwater fishes conserve salts by efficient absorption of ions from the filtrate in the nephrons.
Amphibian’s kidneys function much like those of freshwater fishes. When in fresh water, the skin of the frog accumulates certain salts from the water by active transport, and the kidneys excrete a dilute urine. On land, where dehydration is the most pressing problem of osmoregulation, frogs conserve body fluid by reabsorbing water across the epithelium of the urinary bladder.
Bony fishes that live in seawater, being hyperosmotic to their surroundings, face the opposite problem to that of their freshwater. relatives. In many species, nephrons lack glomeruli and capsules, and a concentrated urine is formed by secreting ions into the renal tubules.
The urine osmolarity and volume in mammals depends on water intake and ambient temperature. The urine osmoic concentration in humans may vary anywhere between 50 to 1400 milliosmoles per liter. Thus, the human kidney has the ability to produce an extremely dilute (hypoosmotic) to a highly concentrated (hyperosmotic) urine depending on the requirements of water conservation in the body.
The urine osmolarity varies in lower vertebrates also to some extent. Thus, a frog in water produces a highly dilute urine in large volume while in dry air, it produces a smaller volume of slightly more concentrated urine with the help of intidiuretic hormone secreted by the hypothalamohypophyseal system.